Non scientifically, the manure is hot and moist. Warmer and contains more moister then the outside ambient air. So the moister that you see is steam. More or less. A steaming pile ;) Posted via Mobile Device
Did you all know that in the years before all the advances in technology, race jockeys used to dig themselves deep into the barns manure pile to "steam" themselves to make weight??
Yeppers, a little pile steams a bit until it cools, but that huge pile 'O poo gets so hot deep inside that it really puts off the heat. Even if the outside air isn't really all that cold, you can usually see steam.